How do you test and debug fetch and promise code in the browser?

1 What are fetch and promise

Fetch is a method that lets you send HTTP requests to a server and receive a response as a promise object. A promise is an object that represents a future value or outcome of an asynchronous operation. Use the then() and catch() methods to handle the fulfillment or rejection of a promise, or the async/await syntax to write more readable code. Fetch and promise are widely supported by modern browsers, but you can also use polyfills or libraries to add compatibility for older browsers.

3 How to debug fetch and promise in the browser

Another useful tool for debugging fetch and promise in the browser is the Sources panel of the developer tools. The Sources panel lets you set breakpoints, step through your code, watch variables, and evaluate expressions. Use it to debug your fetch and promise code in several ways. For example, you can set a breakpoint on the line where you call fetch, and then inspect the request and response objects in the Scope section. You can also set a breakpoint on the line where you handle the promise, and then check the value or state of the promise in the Console section. You can also use the async call stack feature to see the chain of promises that led to the current breakpoint.

4 How to test fetch and promise in the browser

Besides inspecting and debugging your fetch and promise code in the browser, you may also want to test it with different scenarios and conditions. One way to do that is to use mock servers or APIs that simulate the behavior of real servers. Use tools like JSONPlaceholder, Mocky, or Postman to create fake endpoints that return different responses based on your parameters. Then, use these endpoints in your fetch and promise code and see how your code handles different situations, like success, failure, timeout, or redirection.

5 How to improve fetch and promise in the browser

Finally, you may also want to improve your fetch and promise code in the browser by adding some features or optimizations that can enhance its performance or usability. Use the AbortController interface to cancel a fetch request if it takes too long or if the user cancels it. The Cache API can also be used to store and retrieve fetch responses from a cache, which can improve the speed and offline availability of your page. You can also use the Service Worker API to intercept and modify fetch requests and responses, which can enable more advanced scenarios, such as background sync, push notifications, or offline fallback.